It is well known to determine the location of a mobile device based on signals that are transmitted by the mobile device received at a plurality of reference stations. When a location estimation technique is dependent on signal arrival time or signal transit time measurements, it is well recognized that signal multipath is often highly deleterious to the accuracy of the location estimate. Multipath refers to the reflection of a signal, or component of a signal off an object between the transmitter and the receiver.
In the context of a wireless network, a mobile device or mobile station (MS) transmits uplink signals to an access point or base station (BS) that may have a certain functional relationship (in terms of signal timing) with respect to downlink signals transmitted by a serving access point. This functional relationship permits an extraneous system to observe the transmissions between the mobile devices and the access point, and by doing so, determine the time of transmission for a given signal from the mobile device.
Knowing the time of transmission of a signal from a mobile device, and the time at which this same signal is received at a set of other observing stations (termed Wireless Location Sensors or WLSs), it is possible to obtain a set of Range Rings (RRs) representing the distance of the mobile device to each WLS. The intersection of the RRs represents the estimated geographic location of the mobile device.
However, in most location systems, the WLSs do not know the time of transmission of the signals from the mobile device. This is especially so when the mobile device is not synchronized with the access point. Because of this problem, a common location technology uses the time difference of arrival (TDOA) of the signal transmitted by the mobile device at a plurality of WLSs. Each pair of WLSs produces a hyperbola of points representing the possible location of the mobile device. The intersection of the hyperbolae between at least three WLSs provides an estimate of the location of the mobile device. Thus the TDOA technique is useful when the time of transmission of the signal is not known or cannot be determined.
The present application is directed to the novel use of RRs to estimate location referred to as Multiple Range Estimation Location (MREL). Additional embodiments include adjusting the Range Rings to account for errors and mitigate multipath interference.